1. Field of the Invention
This invention is generally related to traffic monitoring systems and, more particularly, to surge protectors including arrestor circuits and related circuit boards which are mounted between telemetering and other monitoring equipment and roadway embedded inductive loops, piezo electric sensors and/or strain gauge weigh bridges.
2. History of the Related Art
There are generally three types of conventional sensors which are used to monitor, count, classify and weigh vehicles traveling on roadways. These basic sensors are known as inductive loop sensors, piezo electric sensors and strain gauge weigh bridges. The sensors are designed to be embedded in, or placed on, a roadway and are connected to monitoring and/or telemetering equipment which receive signals from the sensors and communicate the information received from the sensors to remote locations. Typically, the monitoring and telemetering equipment are mounted in roadside cabinets adjacent the sensors and are either powered by an available source of AC current or by solar powered. The monitoring equipment may also include a modem for communications over a conventional telephone line.
Conventional roadway traffic sensors are sensitive to environmental conditions and therefore various types of surge protectors have been used for isolating the effect of stray signals associated with the grounding systems and for providing protection from high voltage surges from lightning strikes. However, it has been found in many instances that the surge protection devices utilized with conventional systems, and in particular loop traffic sensors, have not been adequate and such devices exhibit an extremely high failure rate especially during seasons of increased electrical storm activity.
Conventional loop sensor circuits include a loop oscillation circuit which is connected in series with inductive loops or coils located in a roadway. Prior protection devices have included isolation transformers mounted between the loop oscillator circuit and the roadway loops to provide for common mode and differential lightning protection, ground isolation and common mode and low frequency noise rejection. In addition, diodes and voltage limiting neon tubes have been used with the transformers to provide some limited protection from surges caused by lightning. See for instance, U.S. Pat. No. 3,943,339.
Unfortunately, such surge protection devices have not proven to be satisfactory. In operation, such surge arrestors function as fuses and are generally completely destroyed when subject to the current surge caused by a lightning strike. Failure is often caused by the inability of the circuit boards to which the components of the surge arrestors are mounted to withstand high voltage surges. The conductive traces associated with such boards are very small, in the range of 0.01 inch to 0.020 inch, and are thus not able to withstand the high current associated with an electrical surge caused by a lightning strike. Further, the use of neon tubes typically employed as voltage limiters in conventional surge protectors are frequently vaporized by lightning surges which have the capability of producing thousands of amperes of current. Upon the failure of the neon tubes, other monitoring equipments can be destroyed by the pass-through voltage caused by high energy surge. Also, the isolation transformers utilized in such systems are generally small printed circuit devices having less than 300 volts of isolation or withstanding capability. Such transformers are frequently destroyed by the pass-through voltage resulting from the destruction of the neon tubes.
In view of the foregoing, conventional surge protectors used with traffic monitoring equipment have not proven to be adequate in actual use to protect circuits from high surges caused by electrical storm activity. This has led to frequent down time of the traffic monitoring stations and the need for repeated expenditures for maintenance, repair and/or replacement of equipment.